1. Field of the Invention
The invention is in the field of nuclear magnetic resonance detection. More specifically, the invention is a method for distinguishing resonance signals within a mixture of substances with different transversal relaxation rates.
2. Background of the Art
NMR relaxation measurements typically employ a static magnetic field to magnetize nuclei into an equilibrium state and an RF magnetic field orthogonal to the static magnetic field to disturb this equilibrium state of nuclear magnetization. The RF magnetic field is typically applied in a form of short pulses that give rise to free induction decay signals (FD) in a nearby NMR antenna. There a two types of relaxation processes related to energy flow and loss of coherence of spin magnetization in a population of nuclei. These relaxation processes are characterized by two relaxation times: T1, the spin-lattice relaxation time, and T2, the spin-spin relaxation time. The spin-lattice relaxation time is the time constant associated with the return of the longitudinal component to its thermal equilibrium state, which is parallel to the static magnetic field. The spin-spin relaxation time is the time constant associated with the decay of the transversal component of nuclear magnetization to zero. If the static magnetic field is in Z-direction of Cartesian coordinates, then the transversal component, inducing signal in the NMR antenna is in the X-Y plane. The net nuclear magnetization in the X-Y plane decays to zero due to reversible and irreversible processes of de-phasing of spin isochromats. The reversible process is a result of macroscopic inhomogeneity of the static magnetic field. This process can be refocused into a spin echo signal by employing a refocusing RF pulse.
A standard sequence of RF pulses used to measure the true transversal relaxation (not related to the macroscopic inhomogeneity of the static magnetic field) is the CPMG sequence. The CPMG sequence is described, for instance, in Experimental Pulse NMR: A Nuts and Bolts Approach by Fukushima and Roeder. This sequence comprises a first excitation RF pulse that tilts the magnetization into X-Y plane followed by a plurality of refocusing RF pulses with the carrier frequency phase shifted by 90 with respect to the excitation pulse. The period of repetition of the plurality of the refocusing pulses is twice the length of time between the center of the excitation pulse and the center of the first refocusing pulse. The spin echo signal, which results from refocusing the spin isochromats, appears between refocusing pulses. The amplitudes of the echoes represent points on a T2 relaxation curve. This curve is then decomposable into exponential terms in order to differentiate between the types of substances present and/or, in the case of a fluid trapped in a porous structure, to characterize the porous media. In a mixture of substances, though, it is almost impossible to differentiate between substances just by the exponential decomposition if the substance with the shorter decay rate is of very small quantity compared to the others. A systematic error in the relaxation curve measurement caused by non-ideality of the CPMG pulse sequence and inhomogeneity of the static magnetic field will most likely exceed the signal from the short relaxation substance.
Another standard NMR technique for measuring parameters of different substances is based on analyzing the FID. In the case of a high ho nogeneity of the static magnetic field (typically a large and expensive magnet is required and a short dead time of the NMR system, a high precision of separation between substances with short and long transversal relaxation can be achieved. An appropriate analysis of the FID to improve accuracy of the separation is described, for example, in U.S. Pat. No. 5,519,319 issued to Smith et al. In many practical situations, the FID signal is defined by the inhomogeneity of the static magnetic field. No differentiation between substances is possible based on FID in this case.
Another known method for measurement of a substance with short relaxation in a mixture is described in U.S. Pat. No. 6,232,778 issued to Speier et al. The method is based on repeating the NMR experiment quickly enough in order to prevent the magnetization vector of a long NMR relaxation substance to recover its longitudinal component and therefore effectively preventing participation in the accumulated signal. Following this procedure, a multi-exponential data analysis is performed in order to filter out the residuals of the long relaxation component.
Another way to increase the precision of NMR based-differentiation between two substances in a mixture is described in U.S. Pat. No. 6,091,242 issued to Hanawa. A first RF pulse is applied to the sample in order to invert the equilibrium magnetization. For both substances, the longitudinal component of the nuclear magnetization starts to evolve toward the equilibrium. Due to difference in the longitudinal relaxation times, it is possible to choose an appropriate wait time after the application of the inversion pulse to get a zero crossing on the longitudinal magnetization curve for one of the substances in the mixture. An excitation pulse applied at the moment of the zero crossing of one substance will not cause any signal for that substance. As a result, even small amount of the other substance can be effectively detected. This method requires prior knowledge of T1 or Tl, distribution for at least one of the substances.
The methods of Speier '778 and Hanawa '242 are ineffective if the substances with different transversal relaxation times have comparable longitudinal relaxation times. This situation occurs in many practical cases. One example is relayed to detection of a small amount of a heavy oil, for example. Herro Negro, having NMR relaxation parameters T2=0.5 ms and T1=40 ms, in the presence of lighter oils or water in a porous media. The substances with longer relaxation rates may have the relaxation times T1=T2 in the range 20-100 ms that are comparable with the T1 of the heavy oil. Another example is found in detecting bone bound water in the human body. The bound water T2 relaxation time in the frequency range 2-10 MHz is typically 0.3-0.7 ms while longitudinal relaxation time T1 is 20-30 ms. The other fluids in the body have T2 and T1 NMR relaxation times in the range 10-200 ms.
Neither shortening the time between repetitions nor selective T1 relaxation can be effective in these cases. Both known techniques will cause the same extent of NMR signal reduction for long T2 relaxation components as for the short T2 relaxation component.
There is a need for a new pulse sequence that enables elimination of the signal from a long relaxation substance without spoiling the relaxation signal from a short relaxation substance, even if the two substances have the same longitudinal relaxation times. The invention disclosed herein addresses this need.